Due to that PEFC has high energy density, PEFC is expected to use in wide fields of, for example, domestic co-generation power source, power source for mobile instruments, power source for automobiles, and a portable auxiliary power source.
In PEFC, a polymer electrolyte membrane functions as an electrolyte for conducting protons, and simultaneously plays a role of a diaphragm for preventing hydrogen or methanol, which is a fuel, and oxygen from being directly mixed. Such a polymer electrolyte membrane requires to, for example, have high ion-exchange capacity as an electrolyte, be electrochemically stable and have low electric resistance because of passing electric current over a long period of time, have high mechanical strength as a membrane, and have low gas permeability to hydrogen or methanol, which is a fuel, and oxygen.
A perfluorosulfonic acid membrane (NAFION, a registered trade mark, a product of du Pont) is generally used as such a polymer electrolyte membrane. However, conventional fluorine-based polymer ion-exchange membranes including NAFION had the problems that although chemical stability is excellent, ion-exchange capacity is low, and further because of insufficient water retention property, drying of the ion-exchange membrane proceeds, resulting in deterioration of proton conductivity. If many sulfonic acid groups are introduced into the membrane as a countermeasure of the problems, membrane strength remarkably decreases due to water retention, and the membrane easily breaks. Thus, it has been a difficult problem to achieve good balance between proton conductivity and membrane strength. Further, a fluorine-based polymer electrolyte membrane such as NAFION is very expensive because synthesis of a fluorine monomer which is a raw material is complicated, and this is a great hindrance in putting PEFC into practical use.
In view of the above, development of high performance polymer electrolyte membrane with low cost is proceeded as a substitute for the fluorine-based polymer electrolyte membrane including NAFION. As one example, JP-A-9-102322 proposes a polymer electrolyte membrane synthesized by introducing styrene monomers into an ethylene tetrafluoroethylene copolymer (ETFE) by radiation grafting reaction, followed by sulfonation.
However, the conventional polymer electrolyte membranes including the above membrane had the problem that output is greatly decreased in a long-term use. The reason for this is that adhesion between an electrode and a polymer electrolyte membrane decreases by a long-term use. In other words, a space generates between the electrode and the polymer electrolyte membrane, and proton conductivity is disturbed at that portion.
As a technique to improve adhesion between a polymer electrolyte membrane and an electrode, JP-A-4-220957 discloses a method of forming unevenness having a size of about 1-5 μm on a surface of a polymer electrolyte membrane by plasma etching treatment, thereby increasing a contact area. However, this method involved the following problems. Although it is possible to increase a contact area to an electrode by forming unevenness on the surface of a polymer electrolyte membrane, when considering a long-term use, stress such as expansion or shrinkage of a membrane is continuously applied to a polymer electrolyte membrane due to change of liquid retention amount and temperature of the membrane. As a result, where the size of unevenness is too large, the membrane breaks starting from particularly depression. Further, in the case of a long-term use, the effect is not sufficient in the point of durability.